pipe(7) Miscellaneous Information Manual pipe(7)
NAME
pipe - overview of pipes and FIFOs
DESCRIPTION
Pipes and FIFOs (also known as named pipes) provide a unidirectional in-
terprocess communication channel. A pipe has a read end and a write
end. Data written to the write end of a pipe can be read from the read
end of the pipe.
A pipe is created using pipe(2), which creates a new pipe and returns
two file descriptors, one referring to the read end of the pipe, the
other referring to the write end. Pipes can be used to create a commu-
nication channel between related processes; see pipe(2) for an example.
A FIFO (short for First In First Out) has a name within the filesystem
(created using mkfifo(3)), and is opened using open(2). Any process may
open a FIFO, assuming the file permissions allow it. The read end is
opened using the O_RDONLY flag; the write end is opened using the
O_WRONLY flag. See fifo(7) for further details. Note: although FIFOs
have a pathname in the filesystem, I/O on FIFOs does not involve opera-
tions on the underlying device (if there is one).
I/O on pipes and FIFOs
The only difference between pipes and FIFOs is the manner in which they
are created and opened. Once these tasks have been accomplished, I/O on
pipes and FIFOs has exactly the same semantics.
If a process attempts to read from an empty pipe, then read(2) will
block until data is available. If a process attempts to write to a full
pipe (see below), then write(2) blocks until sufficient data has been
read from the pipe to allow the write to complete.
Nonblocking I/O is possible by using the fcntl(2) F_SETFL operation to
enable the O_NONBLOCK open file status flag or by opening a fifo(7) with
O_NONBLOCK. If any process has the pipe open for writing, reads fail
with EAGAIN; otherwise—with no potential writers—reads succeed and re-
turn empty.
The communication channel provided by a pipe is a byte stream: there is
no concept of message boundaries.
If all file descriptors referring to the write end of a pipe have been
closed, then an attempt to read(2) from the pipe will see end-of-file
(read(2) will return 0). If all file descriptors referring to the read
end of a pipe have been closed, then a write(2) will cause a SIGPIPE
signal to be generated for the calling process. If the calling process
is ignoring this signal, then write(2) fails with the error EPIPE. An
application that uses pipe(2) and fork(2) should use suitable close(2)
calls to close unnecessary duplicate file descriptors; this ensures that
end-of-file and SIGPIPE/EPIPE are delivered when appropriate.
It is not possible to apply lseek(2) to a pipe.
Pipe capacity
A pipe has a limited capacity. If the pipe is full, then a write(2)
will block or fail, depending on whether the O_NONBLOCK flag is set (see
below). Different implementations have different limits for the pipe
capacity. Applications should not rely on a particular capacity: an ap-
plication should be designed so that a reading process consumes data as
soon as it is available, so that a writing process does not remain
blocked.
Before Linux 2.6.11, the capacity of a pipe was the same as the system
page size (e.g., 4096 bytes on i386). Since Linux 2.6.11, the pipe ca-
pacity is 16 pages (i.e., 65,536 bytes in a system with a page size of
4096 bytes). Since Linux 2.6.35, the default pipe capacity is 16 pages,
but the capacity can be queried and set using the fcntl(2) F_GETPIPE_SZ
and F_SETPIPE_SZ operations. See fcntl(2) for more information.
The following ioctl(2) operation, which can be applied to a file de-
scriptor that refers to either end of a pipe, places a count of the num-
ber of unread bytes in the pipe in the int buffer pointed to by the fi-
nal argument of the call:
ioctl(fd, FIONREAD, &nbytes);
The FIONREAD operation is not specified in any standard, but is provided
on many implementations.
/proc files
On Linux, the following files control how much memory can be used for
pipes:
/proc/sys/fs/pipe-max-pages (only in Linux 2.6.34)
An upper limit, in pages, on the capacity that an unprivileged
user (one without the CAP_SYS_RESOURCE capability) can set for a
pipe.
The default value for this limit is 16 times the default pipe ca-
pacity (see above); the lower limit is two pages.
This interface was removed in Linux 2.6.35, in favor of
/proc/sys/fs/pipe-max-size.
/proc/sys/fs/pipe-max-size (since Linux 2.6.35)
The maximum size (in bytes) of individual pipes that can be set
by users without the CAP_SYS_RESOURCE capability. The value as-
signed to this file may be rounded upward, to reflect the value
actually employed for a convenient implementation. To determine
the rounded-up value, display the contents of this file after as-
signing a value to it.
The default value for this file is 1048576 (1 MiB). The minimum
value that can be assigned to this file is the system page size.
Attempts to set a limit less than the page size cause write(2) to
fail with the error EINVAL.
Since Linux 4.9, the value on this file also acts as a ceiling on
the default capacity of a new pipe or newly opened FIFO.
/proc/sys/fs/pipe-user-pages-hard (since Linux 4.5)
The hard limit on the total size (in pages) of all pipes created
or set by a single unprivileged user (i.e., one with neither the
CAP_SYS_RESOURCE nor the CAP_SYS_ADMIN capability). So long as
the total number of pages allocated to pipe buffers for this user
is at this limit, attempts to create new pipes will be denied,
and attempts to increase a pipe's capacity will be denied.
When the value of this limit is zero (which is the default), no
hard limit is applied.
/proc/sys/fs/pipe-user-pages-soft (since Linux 4.5)
The soft limit on the total size (in pages) of all pipes created
or set by a single unprivileged user (i.e., one with neither the
CAP_SYS_RESOURCE nor the CAP_SYS_ADMIN capability). So long as
the total number of pages allocated to pipe buffers for this user
is at this limit, individual pipes created by a user will be lim-
ited to one page, and attempts to increase a pipe's capacity will
be denied.
When the value of this limit is zero, no soft limit is applied.
The default value for this file is 16384, which permits creating
up to 1024 pipes with the default capacity.
Before Linux 4.9, some bugs affected the handling of the
pipe-user-pages-soft and pipe-user-pages-hard limits; see BUGS.
PIPE_BUF
POSIX.1 says that writes of less than PIPE_BUF bytes must be atomic: the
output data is written to the pipe as a contiguous sequence. Writes of
more than PIPE_BUF bytes may be nonatomic: the kernel may interleave the
data with data written by other processes. POSIX.1 requires PIPE_BUF to
be at least 512 bytes. (On Linux, PIPE_BUF is 4096 bytes.) The precise
semantics depend on whether the file descriptor is nonblocking (O_NON-
BLOCK), whether there are multiple writers to the pipe, and on n, the
number of bytes to be written:
O_NONBLOCK disabled, n <= PIPE_BUF
All n bytes are written atomically; write(2) may block if there
is not room for n bytes to be written immediately
O_NONBLOCK enabled, n <= PIPE_BUF
If there is room to write n bytes to the pipe, then write(2) suc-
ceeds immediately, writing all n bytes; otherwise write(2) fails,
with errno set to EAGAIN.
O_NONBLOCK disabled, n > PIPE_BUF
The write is nonatomic: the data given to write(2) may be inter-
leaved with write(2)s by other process; the write(2) blocks until
n bytes have been written.
O_NONBLOCK enabled, n > PIPE_BUF
If the pipe is full, then write(2) fails, with errno set to EA-
GAIN. Otherwise, from 1 to n bytes may be written (i.e., a "par-
tial write" may occur; the caller should check the return value
from write(2) to see how many bytes were actually written), and
these bytes may be interleaved with writes by other processes.
Open file status flags
The only open file status flags that can be meaningfully applied to a
pipe or FIFO are O_NONBLOCK and O_ASYNC.
Setting the O_ASYNC flag for the read end of a pipe causes a signal (SI-
GIO by default) to be generated when new input becomes available on the
pipe. The target for delivery of signals must be set using the fcntl(2)
F_SETOWN command. On Linux, O_ASYNC is supported for pipes and FIFOs
only since Linux 2.6.
Portability notes
On some systems (but not Linux), pipes are bidirectional: data can be
transmitted in both directions between the pipe ends. POSIX.1 requires
only unidirectional pipes. Portable applications should avoid reliance
on bidirectional pipe semantics.
BUGS
Before Linux 4.9, some bugs affected the handling of the
pipe-user-pages-soft and pipe-user-pages-hard limits when using the fc-
ntl(2) F_SETPIPE_SZ operation to change a pipe's capacity:
(a) When increasing the pipe capacity, the checks against the soft and
hard limits were made against existing consumption, and excluded
the memory required for the increased pipe capacity. The new in-
crease in pipe capacity could then push the total memory used by
the user for pipes (possibly far) over a limit. (This could also
trigger the problem described next.)
Starting with Linux 4.9, the limit checking includes the memory re-
quired for the new pipe capacity.
(b) The limit checks were performed even when the new pipe capacity was
less than the existing pipe capacity. This could lead to problems
if a user set a large pipe capacity, and then the limits were low-
ered, with the result that the user could no longer decrease the
pipe capacity.
Starting with Linux 4.9, checks against the limits are performed
only when increasing a pipe's capacity; an unprivileged user can
always decrease a pipe's capacity.
(c) The accounting and checking against the limits were done as fol-
lows:
(1) Test whether the user has exceeded the limit.
(2) Make the new pipe buffer allocation.
(3) Account new allocation against the limits.
This was racey. Multiple processes could pass point (1) simultane-
ously, and then allocate pipe buffers that were accounted for only
in step (3), with the result that the user's pipe buffer allocation
could be pushed over the limit.
Starting with Linux 4.9, the accounting step is performed before
doing the allocation, and the operation fails if the limit would be
exceeded.
Before Linux 4.9, bugs similar to points (a) and (c) could also occur
when the kernel allocated memory for a new pipe buffer; that is, when
calling pipe(2) and when opening a previously unopened FIFO.
SEE ALSO
mkfifo(1), dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), sock-
etpair(2), splice(2), stat(2), tee(2), vmsplice(2), mkfifo(3), epoll(7),
fifo(7)
Linux man-pages 6.9.1 2024-05-02 pipe(7)
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